1. Field of the Invention
The present invention relates to devices for converting radiation image information carried in a storage layer into an electrical signal sequence.
2. Description of the Prior Art
An apparatus for converting radiation image information which is formed by radiation incident on a storage (or persistent) layer into an electrical signal sequence is generally described, for example, in U.S. Pat. No. 3,859,527. In these types of devices films such as other x-ray exposures or transparent originals are read by a laser beam. The light passing through the original is collected using fiber optics and is supplied to a photomultiplier for conversion into electrical signals, this process sometimes being called cross-sectional conversion. Such devices can also be used for illuminating images recorded in a stimulable phosphor, particularly x-ray pictures. This illumination can also be undertaken by scanning with light, such as a laser beam. The illumination light, as in the transillumination of film images, can be supplied via fiber optics to a photomultiplier and converted into electrical signals.
In conventional photomultipliers, the amplification factor changes in accordance with the changing incident light intensities, which may vary greatly due to the differing transparency of different film images, or differing intensity of stimulable images. A constancy of only one to two percent can thus be achieved with photomultipliers during the scanning of an original. This does not permit contrast enhancements (which may be done using the so-called window technique) to be made, which are desired, for example, in scanning x-ray images so that extremely small differences in optical density can be made visible. For this purpose, the amplification factor must be maintained constant within about 0.1% so that stripe artifacts are avoided. This cannot be achieved with photomultipliers, or can only be achieved with great difficulty, because under the influence of the dynode current, the secondary emission coefficient of the dynodes, and thus the amplification factor, changes. This occurs to an even greater extent as the current increases. If the current is maintained low (low MP high-voltage), for example, under 0.01 .mu.A, the smallest current is only 0.1 .mu.A due to the high dynamic ratio of 1:1000, thus requiring a significant circuit outlay for post-amplification.
In theory, replacing the photomultipliers with photo diodes would avoid fluctuations in the amplification factor, however, due to the large area required for image scanning, a high inverse current would result, leading to a less favorable signal-to-noise ratio. Extremely low intensity signals could not be recorded with such a transducer. In x-ray pictures, however, an evaluation of optical intensities of approximately S=3 is meaningful, which would be lost using a diode array. Moreover, the manufacture of photo diodes having the areas required in conventional converters is difficult because large area sensitivities which are uniform over the entire area are troublesome to manufacture.